[Leetcode 34] 113 Path Sum II

Problem:

Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum.

For example:
Given the below binary tree and sum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

return

[
   [5,4,11,2],
   [5,8,4,5]
]

Analysis:

This is tree's DFS problem. Either can use a recursive function or a stack to finish the traversal.

Time complexity is O(n), where n is the number of nodes in the tree;

Code:

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > pathSum(TreeNode *root, int sum) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<int> stack;
        vector<vector<int> > res;
         
        if (root == NULL) return res;
        
        helper(root, 0, sum, stack, res);
        return res;
    }
    
    void helper(TreeNode *node, int pathSum, int sum, vector<int> path, vector<vector<int> > &res) {
        if (node->left == NULL && node->right == NULL) {
            if (pathSum+node->val == sum) {
                path.push_back(node->val);
                res.push_back(path);
            }
        }
        
        path.push_back(node->val);
        
        if (node->left != NULL)
            helper(node->left, pathSum + node->val, sum, path, res);
        
        if (node->right != NULL)
            helper(node->right, pathSum + node->val, sum, path, res);
    }
};

Attention: